perf(tui): stabilize long-session scrolling

ui-tui/src/lib/viewportStore.ts

@@ -0,0 +1,59 @@
+import type { RefObject } from 'react'
+import { useCallback, useSyncExternalStore } from 'react'
+
+import type { ScrollBoxHandle } from '@hermes/ink'
+
+export interface ViewportSnapshot {
+  atBottom: boolean
+  bottom: number
+  pending: number
+  scrollHeight: number
+  top: number
+  viewportHeight: number
+}
+
+const EMPTY: ViewportSnapshot = {
+  atBottom: true,
+  bottom: 0,
+  pending: 0,
+  scrollHeight: 0,
+  top: 0,
+  viewportHeight: 0
+}
+
+export function getViewportSnapshot(s?: ScrollBoxHandle | null): ViewportSnapshot {
+  if (!s) {
+    return EMPTY
+  }
+
+  const pending = s.getPendingDelta()
+  const top = Math.max(0, s.getScrollTop() + pending)
+  const viewportHeight = Math.max(0, s.getViewportHeight())
+  const scrollHeight = Math.max(viewportHeight, s.getScrollHeight())
+  const bottom = top + viewportHeight
+
+  return {
+    atBottom: s.isSticky() || bottom >= scrollHeight - 2,
+    bottom,
+    pending,
+    scrollHeight,
+    top,
+    viewportHeight
+  }
+}
+
+export function viewportSnapshotKey(v: ViewportSnapshot) {
+  return `${v.atBottom ? 1 : 0}:${v.top}:${v.viewportHeight}:${v.scrollHeight}:${v.pending}`
+}
+
+export function useViewportSnapshot(scrollRef: RefObject<ScrollBoxHandle | null>): ViewportSnapshot {
+  const key = useSyncExternalStore(
+    useCallback((cb: () => void) => scrollRef.current?.subscribe(cb) ?? (() => {}), [scrollRef]),
+    () => viewportSnapshotKey(getViewportSnapshot(scrollRef.current)),
+    () => viewportSnapshotKey(EMPTY)
+  )
+
+  void key
+
+  return getViewportSnapshot(scrollRef.current)
+}